Ultraviolet (UV) radiation from the sun is the main causative agent in the formation of non-melanoma skin cancer, already the most frequent type of cancer in the United States. In addition, the risk for exposure to harmful UVB radiation is rising with the continued destruction of the protective atmospheric ozone layer. The induction of programmed cell death, or apoptosis, is a major cellular protective mechanism for keratinocytes exposed to mutagenic doses of UV radiation. Indeed, inhibition of apoptosis by expressing cell survival genes or inactivation of tumor suppressor genes enhances skin tumor formation. A long term goal of this proposal is to elucidate the signal transduction pathways activated by UV radiation which trigger apoptosis, and how these death effector pathways become altered during multi-stage skin carcinogenesis. Induction of apoptosis by UV radiation involves activation of protein kinase C (PKC) delta, however PKCdelta is often inactivated or down-regulated in neoplastic keratinocytes. To determine the role of PKCdelta in the apoptotic response and malignant transformation of human keratinocytes exposed to UV radiation, we propose to 1) Characterize the distribution of PKCdelta in normal skin and skin tumors, and determine the activation sate of PKCdelta in cultured normal keratinocytes and squamous cell carcinoma cell lines; 2) Characterize the mechanisms by which PKCdelta activation triggers keratinocyte apoptosis by establishing keratinocyte cell lines with inducible expression of activated PKCdelta and analyzing their apoptotic phenotype; 3) Dissect the specific roles of PKCdelta activation in UV radiation-induced apoptosis by analyzing the induction of apoptosis by UV radiation in keratinocytes expressing a dominant/negative PKCdelta; and 4) Determine the effects of PKCdelta inactivation on keratinocyte transformation using in vitro and in vivo transformation assays of keratinocytes which have survived UV irradiation. These studies will help define the molecular mechanisms involved in skin carcinogenesis by UV radiation, and may identify molecular targets for preventive or therapeutic interventions. These studies will also enhance our understanding of basic apoptotic signaling pathways relevant to many normal and disease processes.